The present invention relates to cryostat penetration plugs having a field interruptible cryogenic refrigerator interface.
In the generation of medical diagnostic images in magnetic resonance (MR) imaging, it is necessary to provide a temporally stable and spatially homogeneous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures, provides an advantageous means to produce such a field. Accordingly, for such MR imaging devices, a cryostat is employed. A cryostat contains an innermost chamber in which liquid helium, for example, is used to cool the superconductor materials. The cryostat itself, typically comprises a generally toroidal structure with other nested toroidal structures inside the exterior vessel, to provide the desired vacuum conditions and thermal shielding. Since it is necessary to provide electrical energy to the main magnet coil and to various collection coils employed in MR imaging and to replenish coolant material it is necessary that there be at least one penetration through the cryostat walls.
During magnet operation, superconductive temperatures are maintained by helium evaporation as well as by nitrogen evaporation from a nitrogen vessel surrounding and shielding the inner helium vessel. To reduce the expense of replacing cryogenic fluids, a cryogenic refrigerator can be coupled to the cryostat to remove heat from the cryostat radiation shields and thereby reduce cryogen fluid evaporation. Refrigeration can lower the temperature of the intermediate radiation shield surrounding the helium vessel. Radiation heating between the intermediate shield and the helium vessel varies as the fourth power of the shield temperature so reducing the shield temperature by one half would result in a sixteen fold reduction in heat radiation transfer.
Several methods of interfacing cryogenic refrigerators to cryostats have been proposed. Most of these address the need for good thermal contact while maintaining a reasonable ability to remove and/or service the refrigerator. U.S. Pat. No. 4,279,127 describes a refrigerator using a vertical orientation which allows helium gas stratification if the refrigerator fails or is removed for service. The current invention is directed towards an application requiring a substantially horizontal orientation which prohibits the use of a gas space which would develop convection cells.
The rotating disconnect described in application Ser. No. 859,583 filed on May 5, 1986 and assigned to the instant assignee solves the problem of providing a simple field disconnect and is entirely compatible with some types of refrigeration systems. The refrigeration systems for which the rotating system would work well are those in which the cold head components (valve and motor) are on the same axis as the displacer. Unfortunately, many systems are configured with the drive system perpendicular to the displacer and these refrigeration systems could offer potential advantages with regard to reliability and vibration and isolation from high magnetic field regions. Rotating a cold end refrigeration system with the drive system perpendicular to the displacer, about the axis of the displacer will be difficult due to the large displacements that would occur at the motor.
It is an object of the present invention to provide an interface for a horizontal cryostat penetration insert for a magnetic resonance magnet, coupling a 1, 2 or 3 stage cryogenic refrigerator to the penetration insert.
It is a further object of the present invention to provide an interface with an axial moving disconnect between a cryogenic refrigerator and a cryostat penetration insert that minimizes stress on the cryostat penetration insert radiation shields.
It is still a further object of the present invention to provide an interface that provides an easily activated, field thermal disconnect in the event of a refrigerator failure.
It is another object of the present invention to provide an interface which can accommodate refrigerator cold ends with off-axis drives for the valve and displacer.